Telephones having microphones fitted inside an apertured mouthpiece usually have no discrimination against background noise. Noise cancelling microphones are well known. They are sometimes also referred to as pressure gradient microphones and in their construction a vibratable diaphragm which is actuated by speech or other sounds is arranged such that sound paths are provided to both front and rear sides thereof to reduce the effect of unwanted background noise signals. Prior art microphones of this type are unsuitable to serve as direct replacements for conventional microphones inside the mouthpiece of telephone handsets because they are not adapted to work with existing apertures in the handset mouthpiece.
One solution to this problem is described in British Patent Application No. 8500854, Publication No. GB 2169774A. In this application, a noise cancelling microphone arrangement is disclosed which is constructed and arranged for securing to the external surface of the apertured mouthpiece region of a telephone handset without requiring modification to the handset.